A chain saw is typically provided with a power head and sprocket drive mechanism for engaging and driving a loop of saw chain around a guide bar. A particular make and model of a chain saw power head is typically of a standard design and is intended to adapt to a variety of saw chain types and sizes. Similarly, each of the various saw chain types and sizes are of a standard design (including interconnected side links and center links having depending drive tangs) and are intended to adapt to a variety of chain saw power heads.
The components that provide the adaptation of different saw chain types and sizes to different chain saw power head makes and models are the sprocket and sprocket adapter, i.e. the sprocket assembly. The sprocket has radially projected teeth mated to a specific saw chain. The teeth engage the tangs of the saw chain for driving the saw chain around the guide bar. The type of sprocket contemplated herein is the rim sprocket which also includes circular side walls or rims that, together with the teeth, form pockets that confine the drive tangs. The rims of the sprocket also support the side links of the saw chain and through this support, determines the depth at which the center link drive tangs project down into the pockets. A center opening in the sprocket is provided with grooves for receiving the splines of the adapter which in turn is fit to the drive shaft of the power head.
The sprocket adapter includes a shaft with external splines that fit the grooves in the sprocket opening. Typically each spline on the adapter shaft coincides with a groove in the sprocket which is centered on a tooth of the sprocket (e.g. seven splines for seven sprocket teeth). The tangs on the chain are projected inwardly toward the adapter shaft but between the splines to maximize the effective pocket depth. An adapter cup is fixed to the shaft and is sized to fit the clutch mechanism of a specific chain saw type. It is through the clutch mechanism that the adapter cup and shaft, and ultimately the sprocket and saw chain are driven.
The sprocket and adapter are of little consequence in either weight or cost as compared to the power head and saw chain. Yet they are critical to the function of the chain saw. Unless a proper fit is provided to both the power head and saw chain, the chain saw will not operate properly.
The problem to which the present invention is directed is the relative sizing of the sprocket and adapter to each other and to the saw chain. The problem will be discussed herein generally relative to a seven tooth sprocket for a 0.325-inch pitch saw chain, a common saw chain size.
The 0.325-inch pitch spacing of the chain and the sprocket having seven teeth, together dictate the optimum outer circumference of the sprocket, i.e., the distance around the circular outer edges of the rims on which the side links of the chain are supported. The circumference of the sprocket in turn dictates the rim diameter. The pocket depth radially inwardly of the rim must accommodate the length of the drive tang extended inwardly from the side links. This pocket depth is determined by the adapter configuration to which the sprocket is mounted.
The conventional adapter has a radiused portion between adjacent splines that is the bottom of the pockets and is at a depth (the spacing from the rim's outer edge) that is less than that necessary to fully receive the tangs of the saw chain. This restriction imposed by the adapter configuration generated the requirement for increasing the rim diameter to shift the side links and thus the drive tangs radially outwardly on the sprocket. This, in turn, created a slight misfit as between the saw chain and the sprocket teeth and caused undue wearing of the sprocket and/or adapter. Such wearing has heretofore been tolerated as the only acceptable solution to the interference problem.